Field of the Disclosure
Embodiments of the disclosure relate to an active matrix organic light emitting display, and more particularly, to an organic light emitting display capable of compensating for the degradation of a driving thin film transistor.
Discussion of the Related Art
An active matrix organic light emitting display includes organic light emitting diodes (hereinafter, abbreviated to “OLEDs”) capable of emitting light by itself and has advantages of a fast response time, a high light emitting efficiency, a high luminance, a wide viewing angle, etc.
The OLED serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, a light emitting layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the light emitting layer EML and form excitons. As a result, the light emitting layer EML generates visible light.
The organic light emitting display arranges pixels each including an OLED in a matrix form and adjusts a luminance of the pixels based on a gray scale of video data. Each of the pixels includes a driving thin film transistor (TFT) controlling a driving current flowing in the OLED based on a gate-source voltage, a capacitor for uniformly holding a gate voltage of the driving TFT during one frame, and a switching TFT storing a data voltage in the capacitor in response to a gate signal. The luminance of the pixel is proportional to a magnitude of the driving current flowing in the OLED.
The organic light emitting display has disadvantages in that threshold voltages of the driving TFTs of the pixels are differently changed depending on a formation position due to reasons of a process deviation, etc., or electrical characteristics of the driving TFTs are degraded due to a gate-bias stress increased as a driving time passed. When the electrical characteristics of the driving TFT are degraded, a current characteristic curve of the driving TFT is shifted. Therefore, it is difficult to achieve a desired luminance, and life span of the organic light emitting display is reduced.
To solve these problems, in a related art, as shown in FIG. 1, after a deviation between electrical characteristics of driving TFTs of pixels P, i.e., a deviation between threshold voltages of the driving TFTs of the pixels P is sensed by a driver integrated circuit (IC) DIC, an internal operation is performed. A luminance difference resulting from the deviation between the threshold voltages is compensated by adjusting the pixel data for the implementation of an image with reference to a result of the internal operation.
For example, as shown in FIG. 2, when a positive stress is applied to a gate electrode of the driving TFT for a long time to increase the threshold voltage of the driving TFT from ‘Vth1’ to ‘Vth2’ by ‘φ’, and a current characteristic curve of the driving TFT is right shifted from ‘A’ to ‘B’, a current flowing between drain and source electrodes of the driving TFT is reduced from ‘I1’ to ‘I2’ by ‘ΔI’ under the same conditions. In FIG. 2, ‘Vgs’ denotes a gate-source voltage of the driving TFT. To compensate for a reduction in the current, the related art adopts a method for greatly modulating a data voltage applied to the gate electrode of the driving TFT by an increase amount ‘φ’ of the threshold voltage while holding the current characteristic curve of the driving TFT in a degraded state ‘B’. The positive stress the gate electrode of the driving TFT feels is proportional to a magnitude of an driving voltage as well as a length of an driving time. Thus, in the related art, a magnitude (Vth2+φ) of the data voltage applied to the driving TFT has to gradually increase, so as to compensate for the degradation of the driving TFT. As a result, as shown in FIG. 3, the degradation of the driving TFT is accelerated in a compensation process.
Further, as shown in FIG. 4, a range of the voltage the driver IC DIC can output is previously determined depending on its object. Therefore, when a magnitude of a desired compensation voltage exceeds a compensation voltage range (i.e., 16V−12V=4V) of the driver IC DIC due to the excessive degradation of the driving TFT, it is impossible to compensate for the degradation of the driving TFT. The problem is caused because the degradation characteristic of the driving TFT is not saturated at a time point but continued. Further, the problem is caused because the magnitude of the voltage capable of compensating for the degradation of the driving TFT is limited.
The related art compensation method has the problems of the narrow compensation range and a limitation of the compensation range, and thus it is difficult to solve the non-uniformity of the luminance and the reduction in the life span of the organic light emitting display resulting from the degradation of the driving TFT.